Linio, an e-commerce startup based in Mexico City, couldn’t afford Bain or McKinsey when it needed help pricing new products ranging from vintage wines to billiard tables. So this summer it farmed the work out to Anya Rasulova, a 2013 Wharton School MBA who was looking to make some money before starting a full-time job at EBay. “We had so many things to develop in a short period of time,” says Bernardo Cordero, Linio’s managing director. “It was too complicated to do it all internally.” Cordero paid Rasulova, who worked in management consulting for three years prior to business school, $1,500 for an assignment that took about 35 hours. He estimates a small consulting firm would have charged $20,000.

A growing number of companies are using freelance MBAs to access the same brain power they might find at a top-tier consulting firm. The demand has given rise to online marketplaces that are a cross between executive search agencies and freelance job sites — where the featured contractors are skilled at financial modeling, competitive analysis and marketing.

The services are especially popular with startups, small businesses and enterprises operating on lean budgets.“Companies can choose someone who is hyperspecialized and work with them only for the exact amount of time that they need to,” says Daniel Callaghan, the chief executive officer of MBA & Co., a London-based online staffing service. More than 98% of the 16,000 contractors listed on its website hold a postgraduate degree; 35% have worked at consulting and financial firms. Linio’s Cordero found Rasulova through Skillbridge, a New York-based site started in May by a pair of MBAs fresh out of Wharton. Two-thirds of the 300 freelancers listed on Skillbridge have an advanced degree.

To get listed on these sites, professionals submit their résumés to be vetted. When a company posts a project online, the site comes back with a list of qualified specialists who have been singled out for the job. The marketplaces charge companies a commission from 10% to 20%.

Fees paid to consultants-for-hire vary widely. On HourlyNerd, a service started this year by a group of Harvard MBA students who count Mark Cuban as one of their backers, businesses pay $25 to $75 an hour, according to co-CEO Patrick Petitti. At Skillbridge, the average take-home pay of an active freelancer is about $5,000 a month, says CEO Rajeev Jeyakumar.

A former management consultant, Callaghan started MBA & Co. in London in 2009, when a wave of investment bankers lost their jobs and he realized clients might pay for their skills on an as-needed basis. The company raised $1.3 million from investors this year. Callaghan says the largest fee ever commanded by a contractor through MBA & Co. was more than $500,000, paid by a “large American car company” that he declined to name. On the lower end, he says freelancers make $225 a day.

The number of so-called interim management projects is up 12.7% over the past year, according to the Interim Management Association, a London group that tracks such projects globally. In the U.S., independent workers are expected to make up 50% of the workforce by 2020, up from 30% in 2006, according to data from the Freelancers Union and the Government Accountability Office.

Jeyakumar says 10% to 15% of part-timers listed on Skillbridge are what he calls McKinsey moms — women who left work to care for children and plan to return. Another large contingent are current or recently graduated MBA students who pack in projects around their classes or job search.

In an article in the October issue of the Harvard Business Review, Clayton Christensen, author of The Innovator’s Dilemma about disruptive technologies, argues such marketplaces have the potential to undermine segments of the traditional consulting industry. Derek van Bever, a Harvard Business School colleague of Christensen’s who co-authored the article, says, “We absolutely see HourlyNerd as an example of a larger trend that is going on. Larger firms should see them as evidence of disruption entering their business at the basement level and working its way up.”